Sunscreen compositions and methods and materials for producing the same

ABSTRACT

A method of preparing a sunscreen including a solvent system and a filter system, the method including the step of controlling the polarity of the solvent system to control the rate of photodecay of the filter system, as well as sunscreen compositions and compounds for producing sunscreen compositions, are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention includes a photoactive coating compositionand methods and materials for making a photoactive coating composition.More particularly, the field of the invention includes a sunscreencomposition for use on human skin and methods and materials forformulating a sunscreen composition.

2. Brief Description of Related Technology

It is well known that ultraviolet radiation (light) having a wavelengthfrom about 280 nm or 290 nm to about 320 nm (UV-B) is harmful to humanskin, causing burns that are detrimental to the development of a goodsun tan. UV-A radiation (about 320 nm to about 400 nm), while producingtanning of the skin, also can cause damage, particularly to verylightly-colored or sensitive skin, leading to reduction of skinelasticity and wrinkles. Therefore, a sunscreen composition for use onhuman skin preferably includes both a UV-A and a UV-B filter to preventmost of the sunlight within the full range of about 280 nm or 290 nm toabout 400 nm from damaging human skin.

Ultraviolet radiation from the sun or artificial sources can also causeharm to coatings containing photoactive substances, such as photoactivepigments and dyes, by breaking down chemical bonds in the structure of acomponent such as a polymer, a pigment, or a dye. This photodegradationcan lead to color fading, loss of gloss, and loss of physical andprotective properties of a coating. Photodegradation can take place inseveral steps which include one or more components of a coatingabsorbing UV radiation. The absorbed radiation can excite the absorbingmolecules and raise them to a higher energy level, which can be veryreactive. If the molecule cannot be relaxed, bond cleavage and theformation of free radicals will occur. These free radicals can attackone or more color molecules and/or a polymer backbone and form more freeradicals. UV-A and UV-B filters can also be used to absorb UV radiationto protect a pigmented coating.

The UV-B filters that are most widely used in the U.S. in commercialsunscreen compositions are paramethoxycinnamic acid esters, such as2-ethylhexyl paramethoxycinnamate, commonly referred to as octylmethoxycinnamate or PARSOL MCX, octyl salicylate, and oxybenzone.

The organic UV-A filters most commonly used in commercial sunscreencompositions are the dibenzoylmethane derivatives, particularly4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (also calledavobenzone, sold under the brand name PARSOL 1789). Otherdibenzoylmethane derivatives described as UV-A filters are disclosed inU.S. Pat. Nos. 4,489,057; 4,387,089 and 4,562,067, the disclosures ofwhich are hereby incorporated herein by reference. It is also well knownthat the above described UV-A filters, particularly the dibenzoylmethanederivatives, can suffer in rapid photochemical degradation, when usedalone or when combined with the above-described most commercially usedUV-B filters.

Typically, the above-described UV-B filters are combined with theabove-described UV-A filters in a solution with other lipophilic or oilyingredients. This solution of oily ingredients, known to formulators ofcosmetic products including sunscreens as the “oil phase,” is typically,but not necessarily, dispersed with the help of emulsifiers andstabilizers into an aqueous solution composed primarily of water, tomake an emulsion which becomes a final cream or lotion form of asunscreen composition.

Sunscreen performance has been extremely difficult to predict based onthe levels of sunscreen active compounds in the formulation,particularly when the formulation includes one or more sunscreen activecompounds that suffer from relatively rapid photodegradation, such asavobenzone. Because of this, each formulation has required expensivelaboratory testing to determine the UV absorbance, as a function of time(quantity) of exposure of the formulation to UV radiation.

The formulation of a final sunscreen composition has been achievedlargely through an iterative trial-and-error process, based on knownrules of thumb. A UV filter system (one or more UV-absorbing compounds)is selected to provide UV-A and/or UV-B protection. A solvent system isselected to dissolve the components of the UV filter system. Next, theconcentrations of each component in the filter system dissolved in thesolvent system are selected in an attempt to achieve a sunscreen havinga particular sun protection factor (SPF). If the selected solvent systemis not able to completely dissolve the UV filter system at the desiredconcentrations of UV filter components, then the solvent system must bechanged. For example, a greater amount of one or more solvents can beused (effectively lowering the concentration of the UV filter system),or different solvents can be used in the solvent system. The formulatedsunscreen composition is then tested in panels of human volunteers,e.g., according to a sunscreen monograph (see, e.g., Title 21 of theU.S. Code of Federal Regulations, Part 351) to measure the SPF of thecomposition.

In a SPF test, sunscreen compositions applied to the skin receive dosesof UV energy simulating sun exposure. For example, for a product to belabeled as SPF 30 in the U.S., it must prevent sunburn until a UV doseequivalent to 30 times the minimal erythema dose (MED; 1 MED is about 21mJoule/cm²) is received. This dose is approximately equivalent to a fullday of summer sun exposure.

If the sunscreen composition does not achieve the desired SPF, then thecomposition must be reformulated, typically by adding additionalquantities of UV filters. Again, if the solvent system is not able tocompletely dissolve the UV filter system at the desired concentrationsof UV filter components, then the solvent system must be changed again.The reformulated sunscreen composition containing a higher concentrationof UV filters and, potentially, a different solvent system, is thenre-tested to gauge SPF.

This iterative trial-and-error process is further complicated by thephotodecay of the photoactive compounds in the sunscreen composition,such as UV filters. Sunscreen compositions containing rapidly-degradingphotoactive compounds (e.g., avobenzone) degrade in rapid exponentialfashion when exposed to UV radiation, but start out with SPF valuesseveral-fold higher than their values after dosing with UV radiation.Thus, the SPF rating after testing (e.g., after 30 MED) are thecumulative SPF values over the period of UV irradiation while one ormore compounds may be degrading and losing effectiveness, furthercomplicating the ability to formulate a sunscreen composition with adesired effectiveness.

SUMMARY

One aspect of the invention is a method of preparing a sunscreencomposition including the step of controlling the polarity of a solventsystem in the composition to control the rate of photodecay of a filtersystem in the composition.

Another aspect of the invention is a method of formulating a sunscreencomposition, including the steps of selecting a filter system; preparingparallel mixtures of the filter system in a plurality of analyticalsolvent systems, all of the mixtures having substantially the sameconcentration of filter system; determining the polarity of each of themixtures; determining a rate constant of photodecay of each of themixtures, or sunscreen compositions including the mixtures; selecting afinal solvent system based on its polarity; and mixing the filter systemand the final solvent system.

Yet another aspect of the invention is a composition including a filtersystem and a solvent system, wherein a mixture of the filter system andthe solvent system in the ratio present in the composition has a highpolarity, for example, a composition having a high polarity may have adielectric constant at least about 7.

Still another aspect of the invention is a sunscreen compositionincluding a filter system and a solvent system, wherein the solventsystem includes a amide containing a C₄-C₄₀ hydrocarbon.

Another aspect of the invention is sunscreen composition including afilter system and a solvent system, wherein the solvent system includesa diester of malic acid, wherein the malic acid is esterified with aC₄-C₃₀ hydrocarbon.

Yet another aspect of the invention is a sunscreen composition includinga filter system and a solvent system, wherein the rate constant ofphotodecay of the filter system is about 200% or less of the theoreticalminimum rate constant of photodecay.

Further aspects of the invention may become apparent to those skilled inthe art from a review of the following detailed description, taken inconjunction with the appended claims. While the invention is susceptibleof embodiments in various forms, described hereinafter are specificembodiments of the invention with the understanding that the disclosureis illustrative, and is not intended to limit the invention to thespecific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of rate constant of photodecay versus dielectricconstant for various sunscreen formulations. A curve for a second-orderpolynomial fit to the data is displayed for comparison.

FIG. 2 is a plot of rate constant of photodecay versus dielectricconstant for two additional sunscreen formulations having the samefilter set as used in the data of FIG. 1, overlayed on the data of FIG.1 and the original polynomial fit to the data of FIG. 1, for comparison.

FIG. 3 is a plot of rate constant of photodecay versus dielectricconstant for various sunscreen formulations having another filter set. Acurve for a second-order polynomial fit to the data is displayed forcomparison.

FIG. 4 is a plot of rate constant of photodecay versus dielectricconstant for various sunscreen formulations having yet another filterset. A curve for a second-order polynomial fit to the data is displayedfor comparison.

FIG. 5 is a plot of rate constant of photodecay versus dielectricconstant for two additional sunscreen formulations having the samefilter set as used in the data of FIG. 4, overlayed on the data of FIG.4 and the original polynomial fit to the data of FIG. 4, for comparison.

FIG. 6 is a plot of rate constant of photodecay versus dielectricconstant for various sunscreen formulations having still another filterset. A curve for a second-order polynomial fit to the data is displayedfor comparison.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein are sunscreen compositions and methods of makingsunscreen compositions. It has been found that, quite surprisingly, thestability of a photoactive compound is affected by the polarity of asolution of the compound and, hence, in part based on the polarity ofthe solvent system. Now knowing that the polarity of the solutionaffects the stability, one might expect that the more polar the solutionis, the greater the stability it will impart to the photoactivecompound. In contrast, and even more surprisingly, it has been found(for example, with reference to a specific case) that as the polarity ofa solvent system including a dissolved, rapidly-photodegradable compoundis increased, the rate of photodecay initially decreases—but thenincreases again as the polarity is further increased. Thus, aphotodegradable compound in solution will degrade as a second-orderfunction of the overall polarity of the solution. Currently acceptedphotochemical theory provides the possibility that the mechanism bywhich a photodegradable compound is stabilized is the transfer of aphotonically-excited electron to a nearby molecule of the same ordifferent species (see, e.g., N. J. Turro, Modem MolecularPhotochemistry, Chapter 9, Benjamin/Cummings Publ. Co., Menlo Park,Calif. (1991)), however photochemical theory does not describe theobserved phenomena. Though not intending to be bound by such a belief,the observed phenomena are believed to coincide with the electrontransfer theory of Professor Rudolph A. Marcus of the CaliforniaInstitute of Technology, for which he received the 1992 Nobel Prize inChemistry.

The phenomena described above, coupled with the knowledge that,heretofore, sunscreen compositions have been formulated without specificregard to the relationship between polarity and photostability and, innewly-discovered fact, have had non-optimal polarities, forms the basisfor at least one aspect of the methods and compositions describedherein.

A photoactive compound is one that responds to light photoelectrically.hi preferred compounds and methods disclosed herein, a photoactivecompound is one that responds to UV radiation photoelectrically. Forexample, photoactive compounds that respond to UV radiationphotoelectrically by rapid photodegradation can benefit highly from thecompositions and methods disclosed herein, even though the benefits ofthe compositions and methods disclosed herein are not limited to suchcompounds. Photostability is a potential problem with all UV filtersbecause they are deliberately selected as UV-absorbing molecules. Inother applications, a photoactive compound may be a pigment or a dye(e.g., a hydrophobic dye).

UV filters include compounds selected from the following categories(with specific examples) including: p-aminobenzoic acid, its salts andits derivatives (ethyl, isobutyl, glyceryl esters;p-dimethylaminobenzoic acid); anthranilates (o-aminobenzoates; methyl,menthyl, phenyl, benzyl, phenylethyl, linalyl, terpinyl, andcyclohexenyl esters); salicylates (octyl, amyl, phenyl, benzyl, menthyl(homosalate), glyceryl, and dipropyleneglycol esters); cinnamic acidderivatives (menthyl and benzyl esters, alpha-phenyl cinnamonitrile;butyl cinnamoyl pyruvate); dihydroxycinnamic acid derivatives(umbelliferone, methylumbelliferone, methylaceto-umbelliferone); camphorderivatives (3-benzylidene, 4-methylbenzylidene, polyacrylamidomethylbenzylidene, benzalkonium methosulfate, benzylidene camphor sulfonicacid, and terephthalylidene dicamphor sulfonic acid); trihydroxycinnamicacid derivatives (esculetin, methylesculetin, daphnetin, and theglucosides, esculin and daphnin); hydrocarbons (diphenylbutadiene,stilbene); dibenzalacetone and benzalacetophenone; naptholsulfonates(sodium salts of 2-naphthol-3,6-disulfonic and of2-naphthol-6,8-disulfonic acids); dihydroxy-naphthoic acid and itssalts; o- and p-hydroxydiphenyldisulfonates; coumarin derivatives(7-hydroxy, 7-methyl, 3-phenyl); diazoles (2-acetyl-3-bromoindazole,phenyl benzoxazole, methyl naphthoxazole, various aryl benzothiazoles);quinine salts (bisulfate, sulfate, chloride, oleate, and tannate);quinoline derivatives (8-hydroxyquinoline salts, 2-phenylquinoline);hydroxy- or methoxy-substituted benzophenones; uric and vilouric acids;tannic acid and its derivatives; hydroquinone; benzophenones(oxybenzone, sulisobenzone, dioxybenzone, benzoresorcinol,2,2′,4,4′-tetrahydroxybenzophenone,2,2′-dihydroxy-4,4′-dimethoxybenzophenone, octabenzone,4-isopropyldibenzoyhnethane, butylmethoxydibenzoylmethane, etocrylene,and 4-isopropyl-dibenzoyhnethane).

Particularly useful are: 2-ethylhexyl p-methoxycinnamate, 4,4′-t-butylmethoxydibenzoylmethane, 2-hydroxy-4-methoxybenzophenone, octyldimethylp-aminobenzoic acid, digalloyltrioleate,2,2-dihydroxy-4-methoxybenzophenone, ethyl4-[bis(hydroxypropyl)]aminobenzoate,2-ethylhexyl-2-cyano-3,3-diphenylacrylate, 2-ethylhexylsalicylate,glycerol p-aminobenzoate, 3,3,5-trimethylcyclohexylsalicylate,methylanthranilate, p-dimethylaminobenzoic acid or aminobenzoate,2-ethylhexyl p-dimethylaminobenzoate, 2-phenylbenzimidazole-5-sulfonicacid, 2-(p-dimethylaminophenyl-5-sulfoniobenzoxazoic acid andcombinations thereof.

In one embodiment of the invention a photoactive compound is selectedfrom the group consisting of UV-A filters and UV-B filters, andcombinations thereof. In a cosmetically-acceptable sunscreen embodimentfor use on human skin, a photoactive compound preferably is selectedfrom approved (if regulated), cosmetically-acceptable UV-A filters, UV-Bfilters, and combinations thereof.

For example, for a product marketed in the United States, preferredcosmetically-acceptable photoactive compounds and concentrations(reported as a percentage by weight of the total cosmetic sunscreencomposition) include: aminobenzoic acid (also called para-aminobenzoicacid and PABA; 15% or less), avobenzone (also called butyl methoxydibenzoylmethane; 3% or less), cinoxate (also called 2-ethoxyethylp-methoxycinnamate; 3% or less), dioxybenzone (also calledbenzophenone-8; 3% or less), homosalate (15% or less), menthylanthranilate (also called menthyl 2-aminobenzoate; 5% or less),octocrylene (also called 2-ethylhexyl-2-cyano-3,3 diphenylacrylate; 10%or less), octyl methoxycinnamate (7.5% or less), octyl salicylate (alsocalled 2-ethylhexyl salicylate; 5% or less), oxybenzone (also calledbenzophenone-3; 6% or less), padimate O (also called octyl dimethylPABA; 8% or less), phenylbenzimidazole sulfonic acid (water soluble; 4%or less), sulisobenzone (also called benzophenone-4; 10% or less),titanium dioxide (25% or less), trolamine salicylate (also calledtriethanolamine salicylate; 12% or less), and zinc oxide (25% or less).

Other preferred cosmetically-acceptable photoactive compounds andconcentrations (percent by weight of the total cosmetic sunscreencomposition) include diethanolamine methoxycinnamate (10% or less),ethyl-[bis(hydroxypropyl)]aminobenzoate (5% or less), glycerylaminobenzoate (3% or less), 4-isopropyl dibenzoylmethane (5% or less),4-methylbenzylidene camphor (6% or less), terephthalylidene dicamphorsulfonic acid (10% or less), and sulisobenzone (also calledbenzophenone-4, 10% or less).

For a product marketed in the European Union, preferredcosmetically-acceptable photoactive compounds and concentrations(reported as a percentage by weight of the total cosmetic sunscreencomposition) include: PABA (5% or less), camphor benzalkoniummethosulfate (6% or less), homosalate (10% or less), benzophenone-3 (10%or less), phenylbenzimidazole sulfonic acid (8% or less, expressed asacid), terephthalidene dicamphor sulfonic acid (10% or less, expressedas acid), butyl methoxydibenzoylmethane (5% or less), benzylidenecamphor sulfonic acid (6% or less, expressed as acid), octocrylene (10%or less, expressed as acid), polyacrylamidomethyl benzylidene camphor(6% or less), ethylhexyl methoxycinnamate (10% or less), PEG-25 PABA(10% or less), isoamyl p-methoxycinnamate (10% or less), ethylhexyltriazone (5% or less), drometrizole trielloxane (15% or less),diethylhexyl butamido triazone (10% or less), 4-methylbenzylidenecamphor (4% or less), 3-benzylidene camphor (2% or less), ethylhexylsalicylate (5% or less), ethylhexyl dimethyl PABA (8% or less),benzophenone-4 (5%, expressed as acid), methylene bis-benztriazolyltetramethylbutylphenol (10% or less), disodium phenyl dibenzimidazoletetrasulfonate (10% or less, expressed as acid), bis-ethylhexyloxyphenolmethoxyphenol triazine (10% or less), methylene bisbenzotriazolyltetramethylbutylphenol (10% or less, also called TINOSORB M), andbisethylhexyloxyphenol methoxyphenyl triazine.(10% or less, also calledTINOSORB S).

All of the above-described UV filters are commercially available. Forexample, suitable commercially-available organic UV filters areidentified by trade name and supplier in Table 1 below

TABLE 1 CTFA Name Trade Name Supplier benzophenone-3 UVINULM-40 BASFChemical Co. benzophenone-4 UVINUL MS-40 BASF Chemical Co.benzophenone-8 SPECTRA-SORB American Cyanamid UV-24 DEA-methoxycinnamateBERNEL HYDRO Bernel Chemical ethyl dihydroxypropyl-PABA AMERSCREEN PAmerchol Corp. glyceryl PABA NIPA G.M.P.A. Nipa Labs. homosalateKEMESTER HMS Humko Chemical menthyl anthranilate SUNAROME UVA FeltonWorldwide octocrylene UVINUL N-539 BASF Chemical Co. octyl dimethyl PABAAMERSCOL Amerchol Corp. octyl methoxycinnamate PARSOL MCX BernelChemical PABA PABA National Starch 2-phenylbenzimidazole-5- EUSOLEX 6300EM Industries sulphonic acid TEA salicylate SUNAROME W Felton Worldwide2-(4-methylbenzildene)- EUSOLEX 6300 EM Industries camphorbenzophenone-1 UVINUL 400 BASF Chemical Co. benzophenone-2 UVINUL D-50BASF Chemical Co. benzophenone-6 UVINUL D-49 BASF Chemical Co.benzophenone-12 UVINUL 408 BASF Chemical Co. 4-isopropyl dibenzoylEUSOLEX 8020 EM Industries methane butyl methoxy dibenzoyl PARSOL 1789Givaudan Corp. methane etocrylene UVINUL N-35 BASF Chemical Co.methylene TINOSORB M Ciba Specialty bisbenzotriazolyl Chemicalstetramethylbutylphenol bisethylhexyloxyphenol TINOSORB S Ciba Specialtymethoxyphenyl triazine. Chemicals

A filter system is a set of one or more photoactive compounds. Forexample, a filter system for full-spectrum cosmetic sunscreencomposition can consist of octyl methoxycinnamate and avobenzone.Another suitable filter system can consist of octyl salicylate,benzophenone-3, and avobenzone.

A solvent system includes one or more solvent compounds used to dissolvea filter system. As used herein, a solvent system is the set of allsolvent compound(s) used to dissolve a filter system. Preferably, asolvent system for cosmetic sunscreen compositions will include and/orconsist essentially of lipophilic organic chemicals. When a UV filtercompound is water-soluble (e.g., phenylbenzimidazole sulfonic acid),however, the solvent system can include and/or consist essentially of ahydrophilic chemical.

Preferred solvent compounds include compounds that solubilize one ormore components of a filter system. Suitable solvent compounds include,but are not limited to, C₁₂-C₁₅ alkyl benzoates, capric triglycerides,caprylic triglycerides, diethylhexyl adipate, diethylhexyl malate,diethylhexyl 2,6-naphthalate, ethylhexyl palmitate, ethylhexyl stearate,isoeicosane, isopropyl myristate, isopropyl palmitate, mineral oil,octyldodecyl neopentanoate, polyisobutene, and PPG-2 myristyl etherpropionate. When used with WV filters, such as those identified above,preferred solvent compounds include solvents having even greaterpolarity, such as higher dielectric constants, for example. Thus, whenused with WV filters, such as those identified above, solvent compoundspreferably are selected from the amides, bis-urethanes, and malatesdescribed below, such as bis-(2-ethylhexyl) malate, bis-(2-butyloctyl)malate, isododecyl alcohol dimer with isopherone diisocyanate (IPDI),hexamethylene bis-(2-butyloctyl) urethane, hexamethylenebis-(2-ethylhexyl) urethane, N,N-dimethyldecanamide, andN,N-dipropylisosteramide.

In one embodiment, a preferred solvent compound has a dielectricconstant of at least about 6, preferably about 7.5 to about 12. Inanother embodiment, a solvent compound in admixture with a filter setpreferably will result in a dielectric constant of the mixture at leastabout 7, for example about 7 to about 12. In still another embodiment, asolvent system in admixture with a filter set preferably will result ina dielectric constant of the mixture at least about 8, for example about8 to about 10.

The dielectric constant of a solvent system is a preferred measure ofpolarity of a solvent system, for example because the dielectricconstant is a measure of both inherent and inducible dipole moments.Other measures of polarity include, but are not limited to, the inducedand/or inherent (permanent) dipole moment (e.g., in Debye units), theDimroth-Reichardt E_(T) parameter (see, e.g., McNaught et al., Eds.,IUPAC Compendium of Chemical Terminology, 2nd. Ed. (1997) and K. Dimrothet al., Justus Liebigs Ann. Chem., vol. 661, p. 1-37 (1963) (German)),ionizing power (see, e.g., T. W. Bentley et al., Adv. Phys. Org. Chem.,vol. 14, p. 1-67 (1977); T. W. Bentley et al., Progr. Phys. Org. Chem.,vol. 17, p. 121-158 (1990); see also the Grunwald-Winstein equation inE. Grunwald et al., J. Am. Chem. Soc., vol. 70, p. 846-854 (1948) and A.H. Fainberg et al., J. Am. Chem. Soc., vol. 78, p. 2770-2777 (1956), forexample), Kamlet-Taft solvent parameters (see, e.g., M. J. Kamlet etal., Progr. Phys. Org. Chem., vol. 13, p. 485-630 (1981)), and theZ-value (see, e.g., E. M. Kosower, J. Am. Chem. Soc., vol. 80, p.3253-3260 (1958)). See generally, C. Reichardt, “Solvents and SolventEffects in Organic Chemistry” 2nd ed., Chap. 7: Empirical Parameters ofSolvent Polarity, VCH Publishers, New York, N.Y., (1988).

Mathematically, photodegradation can be described by an exponentialfunction. Thus, Q(a), the absorbance after a radiation dose (i.e.,exposure to a quantity of radiation), can be described by the generalequation (i),

Q(a)=Ae^(−kr)  (i)

wherein A is the original (pre-exposure) absorbance, e is the naturallogarithm base, k is the rate constant of the photodecay, and r is thecumulative dose (e.g., in MED units). Because the absorbance decreasesas the cumulative dose increases (photodecay), the overall term −k willbe negative, and the greater the value of −k (i.e., closer to zero) and,thus, the lower the rate constant of photodecay, the lower is the rateof photodecay. For example, when Q(a) is plotted on a log scale versus ron a linear scale, the function forms a straight line with a slope equalto −k.

Furthermore, it has been found that, for a filter set that includes aphotodegradable compound, the rate constant of photodecay of the filterset can be described as a second-order function of the polarity,preferably the dielectric constant (i.e., relative permittivity) of thefilter set dissolved in the solvent system. Thus, for example, the rateconstant of photodecay of a filter set can be described by the generalequation (ii),

k=−(x∈²+y∈+z)  (ii)

wherein x, y, and z can be empirically determined. The dielectricconstant at the theoretical minimum rate constant of photodecay ∈_(k)_(min) described by formula (iii), $\begin{matrix}{\varepsilon_{k_{\min}} = \frac{\text{-y}}{2x}} & ({iii})\end{matrix}$

wherein x and y are defined as above.

Thus, according to one embodiment of the invention, a method offormulating a sunscreen composition includes the steps of:(a) selectinga filter system including a photoactive compound; (b) selecting aplurality of analytical solvent systems for the filter system; (c)preparing parallel mixtures of the filter system in the plurality ofanalytical solvent systems, all of the mixtures having substantially thesame concentration of filter system; (d) determining the polarity ofeach of the mixtures; (e) determining a rate constant of photodecay ofeach of the mixtures at a selected wavelength; (f) selecting a finalsolvent system based on its polarity; and (g) mixing the filter systemand the solvent system.

The method optionally includes the further step of preparing an emulsionfrom an aqueous system and the mixture of the filter system and thesolvent system. When made, preferably, the emulsion is an oil-in-wateremulsion.

Optionally, a method of formulating an emulsion sunscreen compositioncan include the steps of: (a) selecting a filter system including aphotoactive compound; (b) selecting a plurality of analytical solventsystems for the filter system; (c) preparing parallel mixtures of thefilter system in the plurality of analytical solvent systems, all of themixtures having substantially the same concentration of filter system;(d) determining the polarity of each of the mixtures; (e) selecting anaqueous system for the emulsion; (f) preparing parallel emulsions fromthe aqueous phase and the mixtures; (g) determining the rate constant ofphotodecay of each of the emulsions; (h) selecting a final solventsystem based on its polarity; and (i) preparing an emulsion from thefinal solvent system, the filter system, the aqueous system.Alternatively, this method can be modified such that the polarity ofeach of the parallel emulsions is determined instead of, or in additionto, the parallel mixtures.

A photoactive compound preferably is selected from UV filters, such asthose described above. For example, in a cosmetically-acceptablesunscreen composition, a photoactive compound is selected fromcosmetically-acceptable UV filters. In one embodiment, the filter systemincludes a dibenzoylmethane derivative, preferably avobenzone. In thesame or a different embodiment, the filter system preferably includes aparamethoxycinnamic acid ester.

The analytical solvent systems preferably include a lipophilic organiccompound. The group of analytical solvent systems can be selected basedon the overall polarity of each solvent system, as a result of thepolarities of the individual components of the system. For example, thegroup of analytical solvent systems can be chosen to provide a range ofpolarities of the solvent systems in the group (e.g., by use of one ormore of an amide, a bis-urethane, and a malate described below). When agroup of analytical solvent systems is chosen to provide a broad rangeof polarities of the solvent systems (e.g., with dielectric constantsranging from about 4 to about 10, preferably from about 2 to about 12),then it can be easier to determine the relationship between the polarityand the rate constant of photodecay of the filter system. The group ofanalytical solvent systems can also be chosen to provide a range ofpolarities of the mixtures of the solvent systems with the filtersystem. The above-described selections can be based on the dielectricconstant of each solvent component and/or the dielectric constant ofeach solvent system.

Selecting at least 3 analytical solvent systems allows for a thepossibility of determining the particular function of rate constant ofphotodecay versus polarity. Preferably, at least 5 analytical solventsystems are selected, with varying degrees of polarity. Selecting moreanalytical solvent systems, with varying degrees of polarity, provides abetter chance that rate constants of photodecay will be determined forpolarities on both sides of the vertex (i.e., theoretical point ofminimum rate constant of photodecay).

Next mixtures of the filter system in the plurality of analyticalsolvent systems are prepared, each mixture having substantially the sameconcentration of filter system in a solvent system. By preparing themixtures such that each has substantially the same concentration offilter system (parallel mixtures), the subsequent determination of therate constants of photodecay ensures that the effect of the solventsystem on the rate constant of photodecay is isolated. Theoretically,both the concentration of filter system and the type of solvent systemcan be varied, with a consequence being that much more data would needto be accumulated, to estimate a third-order function (e.g., athree-dimensional surface having a parabolic cross-section), in orderfor such a method to have any predictive value.

According to the general methods described above, next the polarity isdetermined for either each of the mixtures, or parallel emulsionsprepared from each of the mixtures with an aqueous system. As with theparallel mixtures described above, by preparing emulsions such that eachhas substantially the same type and concentration of aqueous system(parallel emulsions), the effect of the solvent system is isolated.

A step of determining the polarity of a mixture or an emulsion can beperformed in various ways. For example, determining a polarity caninclude measuring a property that is a function of polarity, such as adielectric constant. Measurement of a dielectric constant of a liquidcan be performed by various sensors, such as immersion probes,flow-through probes, and cup-type probes, attached to various meters,such as those available from the Brookhaven Instruments Corporation ofHoltsville, N.Y. (e.g., model BI-870) and the Scientifica Company ofPrinceton, N.J. (e.g. models 850 and 870). For consistency ofcomparison, preferably all measurements for a particular filter systemare performed at substantially the same sample temperature, e.g., by useof a water bath. Generally, the measured dielectric constant of asubstance will increase at lower temperatures and decrease at highertemperatures.

When a sunscreen composition produced according to a method disclosedherein includes other components, such as moisturizers, emollients,solvents, co-solvents, lubricants, thickeners, emulsifiers and othercommon cosmetic formulation additives, one or more of which wouldinterfere with an apparatus for measuring polarity, then such acomponent is preferably omitted from the composition tested. Thus, forexample, film-forming polymers or waxy solids such as those commonlyused as emulsifiers, even if used in a final sunscreen formulation,preferably are omitted from the mixture of solvent system and filtersystem which is tested by an apparatus to determine its dielectricconstant, to avoid fouling the dielectric probe. Conversely, othercompositions that would not interfere with such measurements can be, andpreferably are, added to the mixture of solvent system and filter systemprior to measuring polarity.

Determining a polarity can also include retrieving a value of a propertythat is a function of polarity, such as a dielectric constant, from areference document. A reference document is not limited to anyparticular type of document and can include, for example, journalarticles, textbooks, electronic databases, and the like. A referencedocument can also include documentation of prior measurements, such asmeasurements made in practice of a method disclosed herein. As withmeasurements, for consistency of comparison, preferably all valuesretrieved for a particular filter system correspond to substantially thesame sample temperature.

A step of determining the rate constant of photodecay of a mixture or anemulsion also can be performed in various ways. For example, determininga rate constant of photodecay at a selected wavelength can include thesteps of measuring absorbance of the mixture of radiation at theselected wavelength before irradiation and after each of a plurality ofexposures to radiation at the selected wavelength; and then calculatingthe rate constant based on the measurements. Absorbance before and afterexposure to radiation can be measured by various methods, preferably bythe method described in Example 1 below. Other methods are described,for example, in N. Tarras-Wahlberg et al., Changes in UltravioletAbsorption of Sunscreens After Ultraviolet Radiation, J. InvestigativeDermatology, Vol. 113, No. 4, p. 547-553 (October, 1999); R. M. Sayre,Photostability Testing of Avobenzone, Allured's Cosmetics & Toiletriesmagazine, Vol. 114, No. 5, p. 85-91 (May 1999); and “Photostability ofHallStar Photostable SPF 32 Sunscreen Compared to Neutrogena UVA/UVBSunblock SPF 30,” distributed by Suncare Research Laboratories, Memphis,Tenn. (Oct. 5, 2000), the disclosures of which are hereby incorporatedherein by reference.

Preferably, the selected wavelength is in the ultraviolet range. Whenthe filter set includes a relatively rapidly-photodecaying photoactivecompound, then preferably the selected wavelength is in a range ofwavelengths at which the photoactive compound absorbs radiation (e.g.,within the peak range of absorbance).

Determining a rate constant of photodecay can also include retrieving avalue of a rate constant of photodecay from a reference document.

The final solvent system can be selected based on its polarity,including based on a property that is a function of polarity, such as adielectric constant. For example, if the lowest rate constant ofphotodecay of the mixtures or the emulsions described above is achievedwith the use of a particular analytical solvent system (as would beexpected), then that solvent system can be selected as the final solventsystem for preparing the sunscreen composition.

By another option, a second-order function can be fit to the rateconstants of photodecay for the parallel mixtures or emulsions (e.g.rate constant of photodecay as a function of polarity), and a solventsystem can be selected based on that function. For example, a solventsystem can be selected to provide a polarity of the filter systemdissolved in the solvent system corresponding to the theoretical minimumrate constant of photodecay as predicted by the function. The functioncan predict a theoretical minimum rate constant of photodecay, but itmay be the case that a range of polarities correspond to a practicalminimum rate constant of photodecay.

Moreover, it is difficult to predict with accuracy the polarity of amixture of two components, even knowing their individual polarities.Thus, when a filter system has a relatively high polarity, a finalsolvent system can be selected by selecting a final solvent system thathas a polarity that is at least 80% of the polarity at the theoreticalminimum rate constant of photodecay for the filter system. For example,the final solvent system can be selected to provide a dielectricconstant that is at least 80% of the dielectric constant at thetheoretical minimum rate constant of photodecay for the filter system.At least by rough approximation, the relatively high polarity of thefilter system should contribute to the polarity of the mixed system toarrive at a system that has a polarity very close to the polarity at thetheoretical minimum rate constant of photodecay for the filter system.Further refinement can be achieved through titration to a desiredpolarity value.

Thus, one method of method of preparing a sunscreen, the sunscreenincluding a solvent system and a filter system, and the filter systemincluding a photoactive compound, includes the steps of controlling thepolarity of the solvent system to control the rate of photodecay of thefilter system (approximated, for example, by the rate constant ofphotodecay of the filter system). Optionally, the polarity of thesolvent system can be controlled to minimize (e.g., about 150% or lessof the theoretical minimum, preferably about 120% or less) the rate ofphotodecay of the filter system. Preferably, the dielectric constant ofthe solvent system is controlled to minimize the rate of photodecay ofthe filter system.

In similar fashion, a sunscreen composition is disclosed herein, thesunscreen including a selected amount of filter system including aphotoactive compound, and a selected amount of a solvent systemincluding a lipophilic organic compound, wherein the polarity of amixture of the filter system and the solvent system at a concentrationdetermined by the ratio of the selected amounts corresponds to a minimumphotodecay rate for the mixture. As with all methods and compositionsdisclosed herein, the rate of photodecay and any relative value thereof(e.g., a minimum rate of photodecay) can be approximated, for example,by the rate constant of photodecay of the filter system.

The practice of a method disclosed herein, and individual steps thereof,can be performed manually an/or with the aid of electronic equipment.Although processes have been described with reference to particularembodiments, a person of ordinary skill in the art will readilyappreciate that other ways of performing the acts associated with themethods may be used. For example, the order of various of the steps maybe changed without departing from the scope or spirit of the method. Inaddition, some of the individual steps can be combined, omitted, orfurther subdivided into additional steps.

A solvent system includes one or more solvent compounds used to dissolvea filter system. As used herein, a solvent system is the set of allsolvent compound(s) used to dissolve a filter system. Preferred types ofcompounds used as solvents in a solvent system will be highly polarorganic compounds that, in addition to contributing to dissolving aphotoactive compound, will contribute to the overall polarity of the oilphase and/or a mixture of a filter system and a solvent system. Forexample, a preferred solvent will have a high dielectric constantrelative to solvents commonly used in sunscreen compositions.

Preferred compounds suitable for use as solvents in a compositionaccording to the disclosure include, but are not limited to, amides,malates, and bisurethanes, such as those described below.

As used herein, the term “alkyl” includes straight chained and branchedhydrocarbon groups containing the indicated number of carbon atoms,typically methyl, ethyl, propyl, and butyl groups. The term “alkyl” alsoincludes “bridged alkyl,” e.g., a C₄-C₁₆ bicyclic or polycyclichydrocarbon group, for example, norbomyl, adamantyl,bicyclo[2.2.2]octyl, bicyclo[2.2.1]heptyl, bicyclo[3.2.1]octyl ordecahydronaphthyl. The term “cycloalkyl” is defined as a cyclichydrocarbon group, e.g., cyclopropyl, cyclobutyl, cyclohexyl, andcyclopentyl.

As used herein, the term “alkenyl” is defined identically as “alkyl,”except for containing a carbon-carbon double bond. The term“cycloalkenyl” is identical to “cycloalkyl” except containing acarbon-carbon double bond, e.g., cyclopropyl, cyclobutyl, cyclohexyl,and cyclopentyl.

As used herein, the term “aryl,” alone or in combination, is definedherein as a monocyclic or polycyclic aromatic group, preferably amonocyclic or bicyclic aromatic group, e.g., phenyl or naphthyl.

A preferred amide suitable for use as a solvent in a sunscreencomposition according to the disclosure has the structure of formula I,

wherein R¹ is selected from the group consisting of C₁-C₈ alkyl and R²is selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl,aryl, and C₃-C₈ cycloalkyl. Preferably, the solvent is an amide offormula I, wherein R¹ is selected from the group consisting of C₁-C₆alkyl and R² is selected from the group consisting of C₅-C₂₀ alkyl, andC₅-C₂₀ alkenyl. More preferably, the solvent is an amide of formula I,wherein R¹ is selected from the group consisting of C₁-C₄ alkyl and R²is selected from the group consisting of C₈-C₁₅ alky, and C₈-C₁₅alkenyl. Most preferably, the solvent comprises a compound selected fromthe group consisting of N,N-dimethyldecanamide, N,N-diethyldecanamide,N,N-diisopropyldecanamide, N,N-dimethylisosteramide,N,N-diethylisosteramide, N,N-diisopropylisosteramide,N,N-dimethylmyristamide, N,N-diethylmyristamide,N,N-diisopropylmyristamide, and combinations thereof. Preferred amidesof formula I contribute to the solvation of a photoactive compound andan increase in the polarity of the mixture of a solvent system and afilter system. For example, the dielectric constant of a mixture of asolvent system and a filter system preferably is raised by the additionof an amide of formula I to the mixture.

Surprisingly, the compounds of formula I, particularly wherein R₁ isselected from the group consisting of C₁-C₄ alkyl and R² is selectedfrom the group consisting of C₅-C₂₀ alkyl, exhibit much higher thanexpected solvency for crystalline UV filters. Examples of crystalline UVfilters include oxybenzone, avobenzone, and octyl triazone (sold underthe trade name UVINUL T-150 by BASF Corporation) and methylbenzilidenecamphor. Indeed, compared to the traditional solvents used in sunscreencompositions, an amide for which results of solvency testing are shownin Table 2 below N,N-dimethyldecanamide, CAS No. 14433-76-2, EINECS238-405-1) proved to be far superior in its ability to solvatecrystalline UV filters.

Some advantages of improved solvency include: (1) that less solvent canbe used for the same level of one or more UV filters and/or (2) that ahigher concentration of UV filter actives can be dissolved in the samevolume of oil phase, translating into potential cost savings and betteraesthetics.

Thus, other aspects of the invention include the use of an amide ofstructure I, e.g., N,N-dimethyldecanamide, as a solvent for acrystalline UV filter, such as oxybenzone, avobenzone, and octyltriazone, and a sunscreen composition that includesN,N-dimethyldecanarnide and a crystalline UV filter. For example, onesunscreen embodiment can include a low concentration ofN,N-dimethyldecanamide (relative to typical solvent concentrations) anda crystalline UV filter used in atypical concentration. Anothersunscreen embodiment can include N,N-dimethyldecanamide (used at atypical solvent concentration) and a high concentration of a crystallineUV filter or combination of crystalline UV filters (relative to typicalcrystalline UV filter concentrations).

One embodiment of a sunscreen composition can include a solvent systemincluding N,N-dimethyldecanamide and a filter system includingavobenzone, wherein the concentration of avobenzone is greater than 28wt. %, preferably greater than 30 wt. %, for example greater than 35 wt.%. Another embodiment can include a solvent system includingN,N-dimethyldecanamide and a filter system including oxybenzone, whereinthe concentration of oxybenzone is greater than 33 wt. %, preferablygreater than 35 wt. %. Another embodiment can include a solvent systemincluding N,N-dimethyldecanamide and a filter system including octyltriazone, wherein the concentration of octyl triazone is greater than 16wt. %, preferably greater than 20 wt. %, for example greater than 30 wt.%.

TABLE 2 Solvency for Solvency for Solvency for Oxybenzone AvobenzoneOctyl triazone Solvent (wt. %) (wt. %) (wt. %) NN-dimethyldecanamide 38%37% 37% Octocrylene 32% 27%  1% Ethylhexyl 24% 17% 15% methoxycinnamateC₁₂-C₁₅ alkyl benzoates 16% 13.3%    4% Ethylhexyl salicylate 18% 16% 5% Diethylhexyl malate 19% 12.5%    5% Isopropyl myristate 20% 15% <5%Diethylhexyl malate 15% 15% <5% Diethylhexyl adipate 20% 10% <5%

The amides of formula I can be synthesized according to procedures thatare commonly known in the art for the synthesis of amides. For example,a particularly simple procedure for amide synthesis involves thereaction of a primary or secondary amide with the acid halide of acarboxylic acid. See, Zabicky, The Chemistry of Amides, Wiley, New York,N.Y., p. 73-185 (1970).

Another preferred class of compounds suitable for use as solvents in thecomposition according to the disclosure include malates (esters of malicacid) of formula II,

wherein, R³ and R⁴, independently, are selected from the groupconsisting of hydrogen, C₁-C₂₀ alkyl C₁-C₂₀ alkenyl, aryl, and C₃-C₈cycloalkyl. More preferably, a malate is a compound of formula II,wherein R³ and R⁴, independently, are selected from the group consistingof C₁-C₁₀ alkyl, and C₁-C₁₀ alkenyl. Most preferably, the solventcomprises a compound selected from the group consisting of dibutyloctylmalate, diisoamyl malate, and combinations thereof. Preferred malates offormula II contribute to the solvation of a photoactive compound and anincrease in the polarity of the mixture of a solvent system and a filtersystem. For example, the dielectric constant of a mixture of a solventsystem and a filter system preferably is raised by the addition of amalate of formula II to the mixture.

The malates described above can synthesized by techniques that arecommonly known in the art for esterification of acids. For example, theesterification of a carboxylic acid with an alcohol can be accomplishedunder both acid- and base-catalyzed conditions. See, Haslem,Tetrahedron, 36, p. 2409-2433 (1980).

A preferred bisurethane suitable for use as a solvent in a compositionaccording to the disclosure has the structure of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consistingof C₃-C₃₀ alkyl C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈cycloalkenyl. More preferably, a bisurethane will have formula III,wherein R⁵ is selected from the group consisting of C₃-C₂₀ alkyl, C₃-C₂₀alkenyl, aryl, C₅-C₆ cycloalkyl and C₅-C₆ cycloalkenyl, and R⁶ isselected from the group consisting of C₃-C₂₀ alkyl and C₃-C₂₀ alkenyl.For example, a preferred compound of formula III is selected from thegroup consisting of hexamethylene bis(2-butyloctyl) urethane,hexaamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophoroneurethane, bis(dodecyl) tetremethylxylyene urethane,2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane,2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane,bis(2-butyloctyl)-4,4′-biphenylene urethane,bis(2-butyloctyl)-2,4′-biphenylene urethane,bis(2-butyloctyl)-2,2′-biphenylene urethane,bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenyleneurethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinationsthereof. Preferred bisurethanes of formula III contribute to thesolvation of a photoactive compound and an increase in the polarity of amixture of a solvent system and a filter system. For example, thedielectric constant of a mixture of a solvent system and a filter systempreferably is raised by the addition of a bisurethane of formula III tothe mixture.

The bisurethanes described above can be synthesized by the reaction of abisisocyanate with two equivalents of an alcohol, for example. Aprocedure for the synthesis of bisurethanes can be found in U.S. Pat.No. 5,972,324, the disclosure of which is hereby incorporated herein byreference.

In an embodiment wherein the sunscreen composition is an oil-in-wateremulsion, a solvent system preferably is present in an amount from about0.1% to about 40% by weight of the total weight of the composition. Morepreferably, a solvent system of the compositions disclosed herein ispresent in an amount from about 3% to about 20% by weight of the totalweight of the composition. However, in other embodiments, a compositioncan be produced as oily lotions, gels, solids sticks, aerosols, andother forms of cosmetic compositions. In such compositions there mayonly be a minimal aqueous phase, and the solvent system can make upalmost the entire amount of the composition, for example up to about 95%or 99% by weight of the total weight of the composition.

A preferred UV-A filter for use as a photoactive compound in a filtersystem is selected from the group consisting of 2-hydroxy-4-methoxybenzophenone, avobenzone, 4-isopropyl dibenzoylmethane, otherdibenzoylmethane derivatives, menthyl anthranilate, methylenebisbenzotriazolyl tetramethylbutylphenol, bisethylhexyloxyphenolmethoxyphenyl triazine, and combinations thereof. More preferably, aphotoactive compound is selected from the group consisting ofavobenzone, other dibenzoylmethane derivatives, and combinationsthereof.

A preferred UV-B filter for use as a photoactive compound in a filtersystem is selected from the group consisting of 2-ethylhexylp-methoxycinnamate, isoamyl p-methoxycinnamate, octyl salicylate,p-methylbenzylidene-D, L-camphor, sodium2-phenylbenzimidazole-5-sulfonate, sodium 3,4-dimethylphenylglyoxylate,phenylbenzophenone, isooctyl 4-phenylbenzophenone-2′-carboxylate,p-methoxycinnamate, 2-phenyl-5-methylbenzoxazole, octylp-dimethylaminobenzoate, p-aminobenzoic acid, 2-ethylhexylp-dimethylaminobenzoate, pentyl p-dimethylaminobenzoate, ethyl4-bis(hydroxypropyl)-aminobenzoate, 3,3,5-trimethylcyclohexylsalicylate, and combinations thereof. More preferably, a photoactivecompound is selected from the group consisting of octyl salicylate,2-ethylhexyl p-methoxycinnamate, and combinations thereof.

In a composition used on human skin to protect the skin from sunlight, afilter system preferably will include a combination of a UV-A and a UV-Bfilter. For example, the combination of avobenzone and octyl salicylatein a cosmetic sunscreen would typically rapidly photodegrade, but isstabilized in accordance with the compositions described herein (seee.g., Example 2).

The compositions disclosed herein preferably include a filter system inan amount from about 1% to about 40% by weight of the total weight ofthe sunscreen composition. The compositions disclosed herein preferablyinclude a filter system including a photoactive compound wherein aphotoactive compound of the filter system can be present in an amountfrom about 0.1% to about 20% by weight of the total weight of thesunscreen composition. More preferably, a photoactive compound ispresent in the filter system in an amount from about 1% to about 15% byweight of the total weight of the sunscreen composition.

A preferred embodiment of the sunscreen composition includes a filtersystem including a photoactive compound dissolved in a solvent systemthat includes or consists essentially of a lipophilic organic compound,wherein a mixture of the filter system and the solvent system in theratio present in the composition has a dielectric constant of at leastabout 8.

A preferred solvent system of this embodiment includes a malate offormula II described above. Another preferred solvent for thisembodiment can be selected from amides of formula I described above.Most preferably, the solvent system of this embodiment includes acompound selected from the group consisting of N,N-dimethyldecanamide,N,N-diethyldecanamide, N,N-diisopropyldecanamide,N,N-dimethylisosteramide, N,N-diethylisosteramide,N,N-diisopropylisosteramide, N,N-dimethylmyristamide,N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinationsthereof. An amide and/or malate can be combined with a bisurethane offormula III. Preferably, an amide and/or malate used as a solvent in thecompositions disclosed herein is combined with a compound selected fromthe group consisting of hexamethylene bis(2-butyloctyl) urethane,hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophoroneurethane, bis(dodecyl) tetremethylxylyene urethane,2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane,2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane,bis(2-butyloctyl)-4,4′-biphenylene urethane,bis(2-butyloctyl)-2,4′-biphenylene urethane,bis(2-butyloctyl)-2,2′-biphenylene urethane,bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenyleneurethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinationsthereof.

Another embodiment of a sunscreen composition includes a filter systemincluding a photoactive compound, and a solvent system including anamide of formula I. More preferably, the solvent system of thecomposition of this embodiment includes an amide selected from the groupconsisting of N,N-dimethyldecanamide, N,N-diethyldecanamide,N,N-diisopropyldecanamide, N,N-dimethylisosteramide,N,N-diethylisosteramide, N,N-diisopropylisosteramide,N,N-dimethylmyristamide, N,N-diethylmyristamide,N,N-diisopropylmyristamide, and combinations thereof.

Preferably, the solvent system of this embodiment of a sunscreencomposition that includes an amide also includes a bisurethane offormula III. More preferably, the composition of this embodimentincludes a bisurethane selected from the group consisting ofhexamethylene bis(2-butyloctyl) urethane, hexamethylenebis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane,bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluicurethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluicurethane, 2,6-(dodecyl)toluic urethane,bis(2-butyloctyl)-4,4′-biphenylene urethane,bis(2-butyloctyl)-2,4′-biphenylene urethane,bis(2-butyloctyl)-2,2′-biphenylene urethane,bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenyleneurethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinationsthereof. The solvent system of this embodiment can optionally caninclude a malate of formula II described above.

Another embodiment of a sunscreen composition includes a filter systemincluding a photoactive compound and a solvent system including acompound selected from the group consisting of dibutyloctyl malate,diisoamyl malate, and combinations thereof.

The solvent system of this embodiment, optionally, can be supplementedwith an amide of formula I. Preferably, the solvent system includes anamide selected from the group consisting of N,N-dimethyldecanamide,N,N-diethyldecanamide, N,N-diisopropyldecanamide,N,N-dimethylisosteramide, N,N-diethylisosteramide,N,N-diisopropylisosteramide, N,N-dimethylmyristamide,N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinationsthereof.

Optionally, the solvent system of this embodiment that includes a malatecan be supplemented with a bisurethane of formula III, described above.Preferably, the solvent system includes a bisurethane selected from thegroup consisting of hexamethylene bis(2-butyloctyl) urethane,hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophoroneurethane, bis(dodecyl) tetremethylxylyene urethane,2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane,2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane,bis(2-butyloctyl)-4,4′-biphenylene urethane,bis(2-butyloctyl)-2,4′-biphenylene urethane,bis(2-butyloctyl)-2,2′-biphenylene urethane,bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenyleneurethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinationsthereof.

Bis(2-butyloctyl) malate is a malate of formula II, wherein R³ isn-butyl alkyl and R⁴ is n-hexyl alkyl, and is designed to act as asolvent in a composition according to the disclosure. Thus, anotherembodiment is the compound shown in formula II, described asbis(2-butyloctyl)malate. Bis(2-butyloctyl) malate can synthesized bytechniques that are commonly known in the art for the esterification ofacids.

Another embodiment of a sunscreen composition includes a selected amountof a filter system including a photoactive compound, and a selectedamount of a solvent system including a lipophilic organic compound,wherein the rate constant of photodecay of the filter system is about150% or less, preferably about 120% or less, of the theoretical minimumphotodecay rate constant for the filter system.

The solvent system and filter system of this embodiment can include acompound such as an amide of formula I, a malate of formula II, and abisurethane of formula III. One or more of these solvents can contributeto the overall polarity of the composition, as taught herein, such thatthe photodecay rate of the filter system can be minimized to approachthe theoretical minimum photodecay rate for that filter system.

A sunscreen composition according to this embodiment can also include asolvent commonly used in sunscreen compositions, including but notlimited to, isoeicosane, polyisobutene, mineral oil, octyidodecylneopentanoate, ethylhexyl stearate, ethylhexyl palmitate, isopropylpalmitate, isopropyl myristate, C₁₂-C₁₅ alkyl benzoates, caprylictriglyceride, capric triglyceride, and combinations thereof Thus, anunoptimized sunscreen composition can be optimized by the reduction ofthe photodecay rate of the filter system, for example to about 150% orless of the theoretical minimum, by the addition of a highly polarsolvent such as those described herein (see Examples 1-6 below).

Any one of the sunscreen compositions described herein can be combinedwith cosmetically acceptable emollients, stabilizers, emulsifiers, suchas those known in the art, and combinations thereof. These additives canbe used in preparing an emulsion from an aqueous system and the mixtureof the filter system and the solvent system. When made, preferably theemulsion is an oil-in-water emulsion.

Any one of the sunscreen compositions described herein can include asolvent such as those commonly found in cosmetic sunscreen formulations,including but not limited to, a compound selected from the groupconsisting of isoeicosane, polyisobutene, mineral oil, octyldodecylneopentanoate, ethylhexyl stearate, ethylhexyl palmitate, isopropylpalmitate, isopropyl myristate, C₁₂-C₁₅ alkyl benzoates, caprylictriglyceride, capric triglyceride, and combinations thereof.

A mixture of solvent system and filter system, in accordance with onepreferred embodiment, are combined with well-knowncosmetically-acceptable additives, such as moisturizers, emollients,solvents, co-solvents, lubricants, thickeners, emulsifiers and/or othercommon cosmetic formulation additives for solubility of sunscreen activecompounds, emulsification, thickening and to provide other skinenhancement, e.g., moisturizing properties. The compositions can beproduced as oily lotions, gels, solid sticks, emulsions, aerosols, andall other forms of cosmetic compositions. The dielectric constants ofvarious sunscreen additive materials are well known, see, DielectricConstant Reference Guide of ASI Instruments, Inc., pages 1-64,(http://asiinstruments.com/dcl.html), hereby incorporated herein byreference. The dielectric constants of some sunscreen ingredients at 25°C. are shown in Table 3 below.

TABLE 3 Dielectric Constant at 25° C. Isoeicosane (saturatedhydrocarbon) 2.04 Polyisobutene (saturated hydrocarbon) 2.13 Mineral oil2.13 Octyldodecyl neopentanoate 3.04 Ethylhexyl stearate 3.05 Ethylhexylpalmitate 3.06 Isopropyl palmitate 3.20 Isopropyl myristate 3.24 C₁₂-C₁₅alkyl benzoate 3.78 Caprylic/capric triglyceride 3.83 PPG-2 myristylether propionate 3.88 Diethylhexyl adipate 4.21 Diethylhexyl2,6-naphthalate 4.34 Dibutyloctyl malate 4.65 Dioctyl malate 4.74Diethylhexyl malate 4.74 Dioctyl malate 5.78 Diethylhexyl malate 5.88Ethylhexyl methoxycinnamate 5.98 Ethylhexyl salicylate 6.20 Diisoamylmalate 7.42 Isododecyl alcohol dimer with IPDI 10 (estimated.)Isododecyl alcohol dimer with HDI 10 (estimated.) Avobenzone 10(estimated.) Octocrylene 11.08  N,N-dimethyldecanamide 12.43 Benzophenone-3 13    Water 80   

EXAMPLES

The following examples are provided to illustrate the invention but arenot intended to limit the scope of the invention.

Example 1

A series of sunscreen compositions was produced according to the oilphase ingredients and concentrations (formulations) shown in Table 4below.

TABLE 4 Prior Prior ε Art 1 Art 2 A B C D (25° C.) Concentrationsreported as wt % of total formulation octyl salicylate   6.2 5% 5% 5% 5%5% 5% benzophenone-3  13^(†) 3% 3% 3% 3% 3% 3% avobenzone  10* 2% 2% 2%2% 2% 2% saturated   2.04 to 5% 5% 5% hydrocarbons    2.13 isopropylmyristate    3.24 10%  C₁₂-C₁₅ alkyl    3.78 10%  benzoates diethylhexylmalate    5.88 10%  15%  10%  7.5%   isododecyl alcohol  10* 5% dimerw/IPDI NN-   12.43 7.5%   dimethyldecanamide oil phase dielectric 4.564.92  5.89 7.44  7.66  10.59  constant (ε) photodecay rate 0.1  0.0780.04 0.029 0.028 0.038 constant (435%) (339%) (174%) (126%) (122%)(165%) (% of theoretical minimum) *denotes estimated value ^(†)denotesliterature value

The formulations labeled “Prior Art 1” and “Prior Art 2” were formulatedusing typical prior art solvents, in typical concentrations.Formulations “A” though “D” were formulated according to the disclosureherein, and resulted in higher dielectric constants of the oil phases.

For each sunscreen composition, the filter system was blended with thesolvent system to form an oil phase. Next, the dielectric constant ofthe oil phase was measured. Dielectric constant measurements wereperformed with a Scientifica model 850 dielectric constant meter.

Emulsions were created with each oil phase and an aqueous phase. Theaqueous phase was identical for each emulsion. The resulting sunscreenswere tested for photostability by measuring absorbance on a LabsphereUV-1000S Ultraviolet Transmittance Analyzer (software version 1.27)before and after irradiation with a Solar Light Company model 16S solarsimulator (equipped with a WG 320 filter to transmit radiation greaterthan 290 nm) in 5 MED (105 mJ/cm²) increments up to 25 to 35 MEDcumulative dose. Output was monitored by a PMA 2105 UV-B DCS Detector(biologically weighted) and controlled by a PMA 2100 Automatic DoseController (Solar Light Co.).

A synthetic skin substrate was used for testing the sunscreencompositions (VITRO-SKIN substrate by IMS, Inc. of Milford, Conn.). Toprepare the substrate, a 200 g solution of 30 wt. % glycerin and 70 wt.% deionized water was added to an IMS hydrating chamber, and a sheet ofVITRO-SKIN was placed in the hydrating chamber and left overnight(approx. 16 hours). Several 6.5 cm squares were cut from the hydratedVITRO-SKIN and used for absorbance measurements.

To prepare slides for testing, a minimum 100 μl of sunscreen compositionis drawn or placed into a pipe tip (Just or 1100DG, set to dispense 100μl). Using steady, even pressure on the pipettor plunger, the testsubstance was applied to VITRO-SKIN square in a pattern of at least 50small dots arranged to cover a 6 cm center of a square. The VITRO-SKINsquare was then placed on a foam block, and the test material was spreadby finger (covered with a latex glove or finger cot), first in acircular motion, then by a side-to-side motion during which theVITRO-SKIN is deformed by the pressure. The square was then mounted in aslide holder (60 mm×60 mm glassless slide mounts with metal masks byGepe Management AG, Zug, Switzerland) and allowed to dry for 30-60minutes.

To test stability, a slide was positioned on the UV transmittanceanalyzer using registration marks, and a scan of a 1 cm spot on theslide was performed. The slide was then transferred to a holder placedadjacent to the solar simulator and, using a calipers, was positionedsuch that the beam of UV radiation exiting the solar simulatorilluminated the same 1 cm spot on the slide. The following softwaresettings were used: UV-B—290-320 nm; UV-A—320-400 nm; SPF—290-400 nm;Spectral Irradiance; Noon, July 3, Albuquerque, N.Mex.; SPF SpectralIrradiance and Erythermal Effectiveness settings as set by manufacturer.Following an exposure of 5 MED, the slide was again placed in positionon the UV transmittance analyzer, and a scan of the exposed spot wasperformed. The procedure was repeated on the same 1 cm spot on the slideuntil the desired total radiation dosage was achieved.

The absorbance versus cumulative MED data at 370 nm (approximate peakabsorbance for avobenzone) were fit to equation (i), described above, tocalculate the rate constant of photodecay for each formulation.

FIG. 1 plots the rate constant of photodecay versus dielectric constantfor each formulation described in Table 4 above. This Figure shows thatas the dielectric constant of the oil phase was increased, the rateconstant of photodecay approached a minimum and then, quitesurprisingly, increased again.

The data were then fit to a second order polynomial having the form ofgeneral equation (ii)

k=−(x∈²+y∈+z)  (ii)

with the result that x=−0.004215, y=0.072748, and z=−0.33701. Thepolynomial provided a very good fit to the data (strong correlation), asevidenced by a coefficient of determination (R²) of 0.9927. Thus, adefinite, direct relationship exists between the polarity (dielectricconstant) and the rate of photodecay of the filter system. Thesecond-order polynomial curve is displayed on FIG. 1 for comparison.According to equation (iii) described above, the dielectric constant atthe theoretical minimum rate constant of photodecay for this filtersystem is 8.63, and the theoretical minimum rate constant of photodecay(via Formula ii) is 0.023.

Example 2

To test the predictive value of a method of formulating a sunscreencomposition disclosed herein, two additional sunscreen compositions wereproduced using the same filter system as in Example 1, and paralleldielectric constant and rate constant of photodecay measurements weremade. Subsequently, the two new formulations and formulation “C” fromExample 1 above were sent to an independent laboratory for SPF testingof each formulation performed on five human subjects. The sunscreencompositions were produced according to the oil phase ingredients andconcentrations (formulations) shown in Table 5 below. Dielectricconstants and rate constants of photodecay were measured as described inExample 1, and resulted in the values reported in Table 5 below. FormulaC from Example 1 is reproduced for convenience of comparison.

TABLE 5 Prior Art 3 C E Concentrations reported as wt. % ε of totalformulation octyl salicylate   6.2 5% 5% 5% benzophenone-3  13^(†) 3% 3%3% avobenzone  10* 2% 2% 2% C₁₂-C₁₅ alkyl benzoates   3.78 10% octyldodecyl neopentanoate   3.04 5% diethylhexyl malate   5.88 10%isododecyl alcohol dimer  10* 5% w/IPDI diisoamyl malate   7.42 10% dibutyloctyl malate   4.65 5% oil phase dielectric constant 5.48  7.66 7.83  (ε) photodecay rate constant 0.0621 0.0275  0.02367 (% oftheoretical minimum) (269%) (119%) (102%) Average SPF 17.14   20.6  21.3   (% increase over prior art) (+20%) (+24.%) *denotes estimatedvalue

The formulation labeled “Prior Art 3” was formulated using typical priorart solvents, in typical concentrations. Formulations “C” and “E” wereformulated according to the disclosure herein, and resulted in higherdielectric constants of the oil phases.

FIG. 2 plots the rate constant of photodecay versus dielectric constantfor the formulations of Table 5, overlayed on the data of FIG. 1 and theoriginal polynomial fit to the data of FIG. 1, for comparison.

As shown in the figure, the second-order polynomial fit to the originalsix data points was highly predictive of the rate constant of photodecayof the two new formulations “Prior Art 3” and “E.” Also, as expectedbased on the theory described herein, formulations C and E, which hadlow rate constants of photodecay (near the theoretical minimum),provided SPFs determined in independent testing that were much higherthan the prior art formulation.

Example 3

A series of sunscreen compositions having a different filter system wasproduced according to the oil phase ingredients and concentrations(formulations) shown in Table 6 below. Parallel dielectric constant andrate constant of photodecay measurements were made according to themethods ascribed in Example 1.

TABLE 6 Prior Art 4 F G H Concentrations reported as wt. % of ε totalformulation octyl salicylate 6.2   5% 5% 5% 5% avobenzone 10*    2% 2%2% 2% saturated hydrocarbon  2.04 to  5% 2.13 C₁₂-C₁₅ alkyl benzoates3.78 10% diethylhexyl malate 5.88 15% 10% 7.5%   isododecyl alcoholdimer w/HDI 10*   5% N,N-dimethyldecanamide 12.43  7.5%   oil phasedielectric constant (ε) 4.43 6.84 7.33 10.23 photodecay rate constant0.57  0.253  0.186  0.051 (% of theoretical minimum) (1326%) (588%)(433%) (119%) *denotes estimated value

The formulation labeled “Prior Art 4” was formulated using typical priorart solvents, in typical concentrations. Formulations “F” to “H” wereformulated according to the disclosure herein, and resulted in higherdielectric constants of the oil phases.

FIG. 3 plots the rate constant of photodecay versus dielectric constantfor each formulation described in Table 6 above. This Figure shows thatas the dielectric constant of the oil phase was increased, the rateconstant of photodecay unexpectedly approached a minimum in second-orderfashion.

The data were then fit to a second order polynomial having the form ofgeneral equation (ii)

k=−(x∈²+y∈+z)  (ii)

with the result that x=−0.01245, y=0.27179, and z=−1.52649. Thepolynomial provided a very good fit to the data (strong correlation), asevidenced by a coefficient of determination (R²) of 0.9961. Thus, adefinite, direct relationship also exists between the polarity(dielectric constant) and the rate of photodecay of this filter system.The second-order polynomial curve is displayed on FIG. 5 for comparison.According to equation (iii) described above, the dielectric constant atthe theoretical minimum rate constant of photodecay for this filtersystem is 10.92, and the theoretical minimum rate constant of photodecayis 0.043.

Example 4

A series of sunscreen compositions having yet another filter system wasproduced according to the oil phase ingredients and concentrations(formulations) shown in Table 7 below. Parallel dielectric constant andrate constant of photodecay measurements were made according to themethods described in Example 1.

TABLE 7 Prior Art 5 I J K Concentrations reported as wt. % ε of totalformulation octyl salicylate   6.2 5% 5% 5% 5% benzophenone-3  13^(†) 3%3% 3% 3% ethylhexyl methoxycinnamate    5.98 3% 3% 3% 3% avobenzone  10*2% 2% 2% 2% C₁₂-C₁₅ alkyl benzoates    3.78 15%  diethylhexyl malate   5.88 15%  5% diisoamyl malate    7.42 10%  isododecyl alcohol dimerw/HDI  10* 5% N,N-dimethyldecanamide   12.43 10%  oil phase dielectricconstant (ε) 5.77  7.38  8.31  10.85   photodecay rate constant 0.09180.0369 0.0332 0.0835 (% of theoretical minimum) (299%) (120%) (108%)(272%) *denotes estimated value ^(†)denotes literature value

The formulation labeled “Prior Art 5” was formulated using typical priorart solvents, in typical concentrations. Formulations “I” to “K” wereformulated according to the disclosure herein, and resulted in higherdielectric constants of the oil phases.

FIG. 4 plots the rate constant of photodecay versus dielectric constantfor each formulation described in Table 5 above. This Figure shows thatas the dielectric constant of the oil phase was increased, the rateconstant of photodecay approached a minimum and then, quitesurprisingly, increased again.

The data were then fit to a second order polynomial having the form ofgeneral equation (ii)

k=−(x∈²+y∈+z)  (ii)

with the result that x=−0.008795, y=0.147609, and z=−0.65001. Thepolynomial provided a very good fit to the data (strong correlation), asevidenced by a coefficient of determination (R²) of 0.9944. Thus, adefinite, direct relationship also exists between the polarity(dielectric constant) and the rate of photodecay of this filter system.The second-order polynomial curve is displayed on FIG. 6 for comparison.According to equation (iii) described above, the dielectric constant atthe theoretical minimum rate constant of photodecay for this filtersystem is 8.39, and the theoretical minimum rate constant of photodecayis 0.0307.

Example 5

To test the predictive value of a method of formulating a sunscreencomposition disclosed herein, two additional sunscreen compositions wereproduced using the same filter system as in Example 4, and paralleldielectric constant and rate constant of photodecay measurements weremade. Subsequently, the two new formulations were sent to an independentlaboratory for SPF testing of each formulation performed on five humansubjects. The sunscreen compositions were produced according to the oilphase ingredients and concentrations (formulations) shown in Table 8below. Dielectric constants and rate constants of photodecay weremeasured as described in Example 1, and resulted in the values reportedin Table 8 below.

TABLE 8 Prior Art 6 L Concentrations reported as wt % ε of totalformulation octyl salicylate   6.2 5% 5% benzophenone-3  13^(†) 3% 3%ethylhexyl methoxycinnamate    5.98 3% 3% avobenzone  10* 2% 2%octyldodecyl neopentanoate    3.04 5% C₁₂-C₁₅ alkyl benzoates    3.7810%  dibutyloctyl malate    4.65 5% diethylhexyl malate    5.88 10%  oilphase dielectric constant (ε) 5.54  7.65   photodecay rate constant0.0951 0.03675 (% of theoretical minimum) (310%) (120%) Average SPF17.8   22.1    (% increase over prior art) (+24%) *denotes estimatedvalue ^(†)denotes literature value

The formulation labeled “Prior Art 6” was formulated using typical priorart solvents, in typical concentrations. Formulation “L” was formulatedaccording to the disclosure herein, and resulted in higher dielectricconstant of the oil phase.

FIG. 5 plots the rate constant of photodecay versus dielectric constantfor the formulations of Table 8, overlayed on the data of FIG. 4 and theoriginal polynomial fit to the data of FIG. 4, for comparison.

As shown in FIG. 5, the second-order polynomial fit to the original fourdata points was highly predictive of the rate constant of photodecay ofthe two new formulations “Prior Art 6” and “L.” Also, as expected basedon the theory described herein, formulation L, which had a relativelylow rate constant of photodecay, provided an SPF determined inindependent testing that was much higher than that of the prior artformulation.

Example 6

A series of sunscreen compositions having more robust filter systemtypical of a high SPF (e.g., 30+) sunscreen was produced according tothe oil phase ingredients and concentrations (formulations) shown inTable 9 below. Parallel dielectric constant and rate constant ofphotodecay measurements were made according to the methods described inExample 1.

TABLE 9 Prior Art 7 M N Concentrations reported as wt. % ε of totalformulation octyl salicylate   6.2 5% 5% 5% benzophenone-3  13^(†) 4% 4%4% ethylhexyl    5.98 6% 6% 6% methoxycinnamate avobenzone  10* 3% 3% 3%C₁₂-C₁₅ alkyl benzoates    3.78 8% diethylhexyl malate    5.88 8% 3%N,N-dimethyldecanamide   12.43 5% oil phase dielectric 6.91  7.95 10.25   constant (ε) photodecay rate constant 0.0278 0.0177 0.0365 (% oftheoretical (164%) (105%) (216%) minimum) *denotes estimated value^(†)denotes literature value

The formulation labeled “Prior Art 7” was formulated using a typicalprior art solvent, in a typical concentration. Formulations “M” and “N”were formulated according to the disclosure herein, and resulted inhigher dielectric constants of the oil phases.

FIG. 6 plots the rate constant of photodecay versus dielectric constantfor each formulation described in Table 9 above. This Figure shows thatas the dielectric constant of the oil phase was increased the rateconstant of photodecay approached a minimum and then, quitesurprisingly, increased again.

The data were then fit to a second order polynomial having the form ofgeneral equation (ii)

k=−(x∈²+y∈+z)  (ii)

with the result that x=−0.005355, y=0.089286, and z=−0.38908. Thepolynomial provided a very good fit to the data (strong correlation), asevidenced by a coefficient of determination (R²) of 1. Thus, a definite,direct relationship also exists between the polarity (dielectricconstant) and the rate of photodecay of this filter system. Thesecond-order polynomial curve is displayed on FIG. 6 for comparison.According to equation (iii) described above, the dielectric constant atthe theoretical minimum rate constant of photodecay for this filtersystem is 8.34, and the theoretical minimum rate constant of photodecayis 0.0169.

One or more advantages becomes possible by practice of a methoddisclosed herein or by use of a composition described herein. Forexample, financial savings can be achieved by practice of one methoddescribed herein to formulate a new cosmetically-acceptable sunscreencomposition having the same SPF as a prior art composition but using alower concentration of expensive UV filters. Alternatively, a high-SPFcosmetic sunscreen composition can be formulated withingovernment-mandated limits on the use of particular UV filters, bychoosing a solvent or solvent blend disclosed herein. As anotherexample, a known sunscreen formulation can be modified to be morephotostable and, thus, have a higher SPF, by simply adding orsubstituting a highly polar solvent, such as those described herein, fora solvent in the sunscreen formulation that has a relatively lowpolarity. By practice of a method described herein, valuable time andresources can be saved by formulating a sunscreen to have a desired SPF,without the use of trial-and-error experimentation. Further aspects andadvantages of the invention will become apparent to those of ordinaryskill in the art in view of the disclosure herein.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the invention may be apparent to thosehaving ordinary skill in the art.

What is claimed is:
 1. A sunscreen composition, comprising a filter system consisting essentially of one or more photoactive compounds selected tom the group consisting of p-aminobenzoic acid and salts and derivatives thereof; anthranilate and derivatives thereof; salicylate and derivatives thereof; cinnamic acid and derivatives thereof; dihydroxycinnamic acid and derivatives thereof camphor and derivatives thereof; trihydroxycinnamic acid and derivatives thereof, dibenzalacetone naptholsulfonate and salts and derivatives thereof; benzalacetophenone naptholsulfonate and salts and derivatives thereof; dihydroxy-naphthoic acid and salts thereof; o-hyoxydiphenyldisulfonate and salts and derivatives thereof; p-hyoxydiphenyldisulfonate and salts and derivatives thereof; cournarin and derivatives thereof; diazoles derivatives; quinine derivatives and salts thereof, quinoline derivatives; hydroxy-substituted benzophenone derivatives; methoxy-substituted benzophenone derivatives; uric acid derivatives; vilouric acid derivatives; tannic acid and derivatives thereof; hydroquinone; benzophenone derivatives; and dibenzoylmethane derivatives, dissolved in a solvent system comprising a lipophilic organic compound, wherein a mixture of said filter system and said solvent system in the ratio present in said composition has a dielectric constant of at least about
 8. 2. The composition of claim 1, wherein said photoactive compound is selected from the group consisting of 2-hydroxy-4-methoxy benzophenone, avobenzone, 4-isopropyl dibenzoylmethane, menthyl anthranilate, methylene bisbenzotriazolyl tetramethylbutylphenol, bisethylhexyloxyphenol methoxyphenyl triazine, and combinations thereof.
 3. The composition of claim 2, wherein said filter system comprises avobenzone.
 4. The composition of claim 3, wherein said filter system further comprises octyl salicylate.
 5. The composition of claim 2, further comprising a photoactive compound selected from the group consisting of 2-ethylhexyl p-methoxycinnamate, isoamyl p-methoxycinnamate, octyl salicylate, p-methylbenzylidene-D, L-camphor, sodium 2-phenylbenzimidazole-5-sulfonate, sodium 3,4-dimethylphenylglyoxylate, phenylbenzophenone, isooctyl 4-phenylbenzophenone-2′-carhoxylate, p-methoxycinnamate, 2-phenyl-5-methylbenzoxazole, octyl p-dimethylaminobenzoate, p-aminobenzoic acid, 2-ethylhexyl p-dimethylaminobenzoate, pentyl p-dimethylaminobenzoate, ethyl 4-bis(hydroxypropyl)-aminobenzoate, 3,3,5-trimethylcyclohexyl salicylate, and combinations thereof.
 6. The composition of claim 1, wherein said photoactive compound comprises from about 0.1% to about 20% by weight of the total weight of the composition.
 7. The composition of claim 6, wherein said photoactive compound comprises from about 1% to about 15% by weight of the total weight of the composition.
 8. The composition of claim 1, wherein said filter system comprises about 1% to about 40% by weight of the total weight of said composition.
 9. The composition of claim 1, wherein said solvent system comprises a compound of formula I:

wherein R¹ is selected from the group consisting of C₁-C₈ alkyl and R² is selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl and C₃-C₈ cycloalkyl.
 10. The composition of claim 9, wherein said compound of formula I is selected from the group consisting of N,N-dimethyldecanamide, N,N-diethyldecanamide, N,N-diisopropyldecanamide, N,N-dimethylisosteramide, N,N-diethylisosteramide, N,N-diisopropylisosteramide, N,N-dimethylmyristamide, N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinations thereof.
 11. The composition of claim 9, wherein said solvent system comprises a compound of formula II,

wherein, R³ and R⁴, independently, are selected from the group consisting of hydrogen, C₁-C₂₀ alkyl, C₁-C₂₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 12. The composition of claim 11, wherein said compound of formula II is selected from the group consisting of diisoamyl malate, dibutyloctyl malate, and combinations thereof.
 13. The composition of claim 9, wherein said solvent system comprises a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 14. The composition of claim 13, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 15. The composition of claim 11, wherein said solvent system comprises a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 16. The composition of claim 15, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2 ′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 17. The composition of claim 1, wherein said solvent system comprises from about 0.1% to about 40% by weight of the total weight of said composition.
 18. The composition of claim 17, wherein said solvent system comprises from about 3% to about 20% by weight of the total weight of said composition.
 19. A sunscreen composition, comprising a filter system consisting essentially of one or more photoactive compounds selected from the group consisting of p-aminobenzoic acid and salts and derivatives thereof; anthranilate and derivatives thereof; salicylate and derivatives thereof; cinnamic acid and derivatives thereof; dihydoxycinnamic acid and derivatives thereof camphor and derivatives thereof; trihydroxycinnamic acid and derivatives thereof; dibenzalacetone naptholsulfonate and salts and derivatives thereof, benzalacetophenone naptholsulfonate and salts and derivatives thereof; dihydroxy-naphtoic acid and salts thereof; o-hyoxydiphenyldisulfonate and salts and derivatives thereof; p-hyoxydiphenyldisulfonate and salts and derivatives thereof; cournarin and derivatives thereof; diazoles derivatives; quinine derivatives and salts thereof; quinoline derivatives; hydroxy-substituted benzophenone derivatives; methoxy-substituted benzophenone derivatives; uric acid derivatives; vilourio acid derivatives; tannic acid and derivatives thereof; hydroquinone; benzophenone derivatives; and dibenzoylmethane derivatives, and a solvent system comprising a compound of formula I:

wherein R¹ is selected from the group consisting of C₁-C₈ alkyl and R² is selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 20. The composition of claim 19, wherein said compound of formula I is selected from the group consisting of N,N-dimethyldecanamide, N,N-diethyldecanamide, N,N-diisopropyldecanamide, N,N-dimethylisosteramide, N,N-diethylisosteramide, N,N-diisopropylisosteramide, N,N-dimethylmyristamide, N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinations thereof.
 21. The composition of claim 19, wherein said photoactive compound is selected from the group consisting of 2-hydroxy-4-methoxy benzophenone, avobenzone, 4-isopropyl dibenzoyhnethane, menthyl anthranilate, methylene bisbenzotriazolyl tetramethylbutylphenol, bisethylhexyloxyphenol methoxyphenyl triazine, and combinations thereof.
 22. The composition of claim 21, wherein said filter system comprises avobenzone.
 23. The composition of claim 22, wherein said filter system further comprises octyl salicylate.
 24. The composition of claim 21, further comprising a photoactive compound selected from the group consisting of 2-ethylhexyl p-methoxycinnamate, isoamyl p-methoxycinnamate, octyl salicylate, p-methylbenzylidene-D, L-camphor, sodium 2-phenylbenzimidazole-5-sulfonate, sodium 3,4-dimethylphenylglyoxylate, phenylbenzophenone, isooctyl 4-phenylbenzophenone-2′-carboxylate, p-methoxycinnamate, 2-phenyl-5-methylbenzoxazole, octyl p-dimethylaminobenzoate, p-aminobenzoic acid, 2-ethylhexyl p-dimethylaminobenzoate, pentyl p-dimethylaminobenzoate, ethyl 4-bis(hydroxypropyl)-aminobenzoate, 3,3,5-trimethylcyclohexyl salicylate, and combinations thereof.
 25. The composition of claim 19, wherein said photoactive compound comprises from about 0.1% to about 20% by weight of the total weight of the composition.
 26. The composition of claim 25, wherein said photoactive compound comprises from about 1% to about 15% by weight of the total weight of the composition.
 27. The composition of claim 19, wherein said filter system comprises about 1% to about 40% by weight of the total weight of said composition.
 28. The composition of claim 19, further comprising a compound of formula II,

wherein, R³ and R⁴, independently, are selected from the group consisting of hydrogen, C₁-C₂₀ alkyl C₁-C₂₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 29. The composition of claim 28, wherein said compound of formula II is selected from the group consisting of diisoamyl malate, dibutyloctyl malate, and combinations thereof.
 30. The composition of claim 19, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 31. The composition of claim 30, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 32. The composition of claim 28, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 33. The composition of claim 32, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 34. The composition of claim 19, wherein said solvent system comprises from about 0.1% to about 40% by weight of the total weight of said composition.
 35. The composition of claim 34, wherein said solvent system comprises from about 3% to about 20% by weight of the total weight of said composition.
 36. The composition of claim 19, further comprising a compound selected from the group consisting of cosmetically acceptable emollients, stabilizers, emulsifiers, and combinations thereof.
 37. The composition of claim 19, further comprising a compound selected from the group consisting of isoeicosane, polyisobutene, mineral oil, octyldodecyl neopentanoate, ethylhexyl stearate, ethylhexyl palmitate, isopropyl palmitate, isopropyl myristate, C₁₂-C₁₅ alkyl benzoates, caprylic triglyceride, capric triglyceride, and combinations thereof.
 38. A sunscreen composition comprising a filter system comprising a photoactive compound, and a solvent system comprising a compound selected from the group consisting of dibutyloctyl malate, diisoamyl malate, and combinations thereof.
 39. The composition of claim 38, wherein said photoactive compound is selected from the group consisting of 2-hydroxy-4-methoxy benzophenone, avobenzone, 4-isopropyl dibenzoylmethane, menthyl anthranilate, methylene bisbenzotriazolyl tetramethylbutylphenol, bisethylhexyloxyphenol methoxyphenyl triazine, and combinations thereof.
 40. The composition of claim 39, wherein said filter system comprises avobenzone.
 41. The composition of claim 40, wherein said filter system further comprises octyl salicylate.
 42. The composition of claim 39, further comprising a photoactive compound selected from the group consisting of 2-ethylhexyl p-methoxycinnamate, isoamyl p-methoxycinnamate, octyl salicylate, p-methylbenzylidene-D, L-camphor, sodium 2-phenylbenzimidazole-5-sulfonate, sodium 3,4-dimethylphenylglyoxylate, phenylbenzophenone, isooctyl 4-phenylbenzophenone-2′-carboxylate, p-methoxycinnamate, 2-phenyl-5-methylbenzoxazole, octyl p-dimethylaminobenzoate, p-aminobenzoic acid, 2-ethylhexyl p-dimethylaminobenzoate, pentyl p-dimethylaminobenzoate, ethyl 4-bis(hydroxypropyl)-aminobenzoate, 3,3,5-trimethylcyclohexyl salicylate, and combinations thereof.
 43. The composition of claim 38, wherein said photoactive compound comprises from about 0.1% to about 20% by weight of the total weight of the composition.
 44. The composition of claim 43, wherein said photoactive compound comprises from about 1% to about 15% by weight of the total weight of the composition.
 45. The composition of claim 38, wherein said filter system comprises about 1% to about 40% by weight of the total weight of said composition.
 46. The composition of claim 38, further comprising a compound of formula I,

wherein R¹ is selected from the group consisting of C₁-C₈ alkyl and R² is selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 47. The composition of claim 46, wherein said compound of formula I is selected from the group consisting of N,N-dimethyldecanamide, N,N-diethyldecanamide, N,N-diisopropyldecanamide, N,N-dimethylisosteramide, N,N-diethylisosteramide, N,N-diisopropylisosteramide, N,N-dimethylmyristamide, N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinations thereof.
 48. The composition of claim 38, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 49. The composition of claim 48, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 50. The composition of claim 46, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 51. The composition of claim 50, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 52. The composition of claim 38, wherein said solvent system comprises from about 0.1% to about 40% by weight of the total weight of said composition.
 53. The composition of claim 52, wherein said solvent system comprises from about 1% to about 20% by weight of the total weight of said composition.
 54. The composition of claim 38, further comprising a compound selected from the group consisting of cosmetically acceptable emollients, stabilizers, emulsifiers, and combinations thereof.
 55. The composition of claim 38, further comprising a compound selected from the group consisting of isoeicosane, polyisobutene, mineral oil, octyldodecyl neopentanoate, ethylhexyl stearate, ethylhexyl palmitate, isopropyl palmitate, isopropyl myristate, C₁₂-C₁₅ alkyl benzoates, caprylic triglyceride, capric triglyceride, and combinations thereof.
 56. A malate ester of the structure of formula II,

wherein R³ is n-butyl alkyl and R⁴ is n-hexyl alkyl.
 57. A sunscreen composition, comprising a selected amount of a filter system comprising a photoactive compound, and a selected amount of a solvent system comprising a lipophilic organic compound, wherein the rate constant of photodecay of said filter system is about 150% or less of the theoretical minimum rate constant of photodecay.
 58. The composition of claim 57, wherein said filter system comprises avobenzone.
 59. The composition of claim 58, wherein said filter system further comprises octyl salicylate.
 60. The composition of claim 57, wherein said solvent system comprises a compound of formula I:

wherein R¹ is selected from the group consisting of C₁-C₈ alkyl and R² is selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 61. The composition of claim 60, wherein said compound of formula I is selected from the group consisting of N,N-dimethyldecanamide, N,N-diethyldecanamide, N,N-diisopropyldecanamide, N,N-dimethylisosteramide, N,N-diethylisosteramide, N,N-diisopropylisosteramide, N,N-dimethylmyristamide, N,N-diethylmyristamide, N,N-diisopropylmyristamide, and combinations thereof.
 62. The composition of claim 57, wherein said solvent system comprises a compound of formula II,

wherein, R³ and R⁴, independently, are selected from the group consisting of hydrogen, C₁-C₂₀ alkyl C₁-C₂₀ alkenyl, aryl, and C₃-C₈ cycloalkyl.
 63. The composition of claim 62, wherein said compound of formula II is selected from the group consisting of dibutyloctyl malate, diisoamyl malate, and combinations thereof.
 64. The composition of claim 60, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl, C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 65. The composition of claim 64, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 66. The composition of claim 62, further comprising a compound of formula III,

wherein R⁵ and R⁶, independently, are selected from the group consisting of C₃-C₃₀ alkyl C₃-C₃₀ alkenyl, aryl, C₃-C₈ cycloalkyl, and C₃-C₈ cycloalkenyl.
 67. The composition of claim 66, wherein said compound of formula III is selected from the group consisting of hexamethylene bis(2-butyloctyl) urethane, hexamethylene bis(2-ethylhexyl) urethane, bis(dodecyl) isophorone urethane, bis(dodecyl) tetremethylxylyene urethane, 2,4-(2-butyloctyl)toluic urethane, 2,6-(2-butyloctyl)toluic urethane, 2,4-(dodecyl)toluic urethane, 2,6-(dodecyl)toluic urethane, bis(2-butyloctyl)-4,4′-biphenylene urethane, bis(2-butyloctyl)-2,4′-biphenylene urethane, bis(2-butyloctyl)-2,2′-biphenylene urethane, bis(dodecyl)-4,4′-biphenylene urethane, bis(dodecyl)-2,4′-biphenylene urethane, bis(dodecyl)-2,2′-biphenylene urethane, and combinations thereof.
 68. The composition of claim 57, further comprising a compound selected from the group consisting of cosmetically acceptable emollients, stabilizers, emulsifiers, and combinations thereof.
 69. The composition of claim 57, further comprising a compound selected from the group consisting of isoeicosane, polyisobutene, mineral oil, octyldodecyl neopentanoate, ethylhexyl stearate, ethylhexyl palmitate, isopropyl palmitate, isopropyl myristate, C₁₂-C₁₅ alkyl benzoates, caprylic triglyceride, capric triglyceride, and combinations thereof. 